Winter survival shaped by forage abundance and snow depth for a long-distance migratory ungulate

Abstract Adult survival is a crucial factor influencing the population dynamics of caribou ( Rangifer tarandus ), yet its relationship with environmental variables remains poorly understood. We explored linkages among lichen abundance (the primary winter forage), snow depth, and overwinter survival of adult female caribou in the Western Arctic Herd (WAH), a large population of barren-ground caribou ( R. t. granti ) in northwest Alaska. We hypothesized that caribou wintering in areas with greater lichen cover would experience higher survival rates, and that snow depth would moderate this relationship. Using GPS telemetry data from 1,067 caribou winters between 2009 and 2024, we investigated overwinter survival as a function of lichen cover and snow depth. Higher lichen cover was significantly associated with reduced overwinter mortality hazard, but this relationship was strongly and non-linearly affected by snow depth. At intermediate snow depths, caribou in low-lichen habitats experienced 9.1 times higher mortality risk compared to high-lichen areas. However, in deep snow conditions the survival benefits of lichen diminished substantially, suggesting that deep snow may limit forage accessibility. Our results underscore the importance of lichen-rich habitats for barren-ground caribou winter survival and highlight the complex interplay between forage accessibility and snow conditions. Arctic-wide declines in lichen cover could pose a threat to barren-ground caribou populations, with potential long-term demographic consequences, as our study suggests that lichen abundance plays an important role in determining the overwinter survival of adult female caribou.